High-voltage underground power distribution circuits are configured in a number of different configurations. Often a single-phase underground distribution system is configured as a fused radial-circuit (see FIG. 1) or as a fused loop-circuit (see FIG. 2). The distribution circuit could also be configured in a more complex configuration consisting of branches (see FIG. 3). A branched circuit could take on any form.
A transient waveform is generated at the fault site on the initiation of a cable fault. This transient waveform or traveling wave, starts at the fault site and travels in both directions away from the fault. The traveling waves in either section of the circuit reflect between the fault location and a significant change of impedance in the circuit. This significant change of impedance can for example, result from the circuit open-points (end of circuit), the short-circuit caused by the fault, and by transitions from underground to overhead cables. The transition from underground to overhead presents an impedance that is sufficient to essentially block the fault signal, and therefore giving the appearance of a open-point.
A fault recorder, located at the open-point of the circuits illustrated in FIG. 1 and 2, would measure the traveling wave that reflects between the fault and open-point. This traveling wave reflects between the fault and open-point until its energy dissipates. If the circuits in FIGS. 1 and 2 are fed by overhead power lines, a fault recorder in the cable section between the fuse and fault, would record the traveling waves that reflect between the fault and transition point from underground to overhead.
When a circuit is branch as shown in FIG. 3, the fault signal will take different paths depending on the fault site and the branching locations. Should a fault occur on branch segment C, a fault recorder at the open end of branch segment C will record the traveling wave that reflects between the fault and open end of segment C. The fault in segment C, also generates a traveling wave that travels toward segment A. When this traveling wave reaches the intersection of segments A, B and C, the traveling wave splits and continues onto segments A and B. This split traveling wave reflects off the open end of segment B, and off a significant impedance change along segment A. The reflected wave from the open end of segment B splits at the intersection of segments A, B and C. This split traveling wave reflects off the fault in segment C, and off a significant change in impedance on segment A. Likewise, the traveling wave that reflects from the significant impedance change on segment A will split when it reaches the intersection of segments A, B and C, and continue onto segments B and C. The traveling wave created by the fault in segment C seen on segments A and B is therefore a combination of -multiple reflections off the fault and significant impedance changes in segments A and B. A fault recorder at the open end of segment B would therefore record a complex wave shape that consists of multiple reflections.
Should a fault occur in segment B of the branched circuit shown in FIG. 3, a fault recorder located at the open end of segment B would record the traveling wave that reflects between the fault and open end of segment B. Likewise, a fault recorder located at the open end of segment C would record a combination of reflections from the fault and significant changes in impedance on segments A and C. This process would be similar to that described in the previous paragraph.
The fault in segment C would create a simple traveling wave that reflects between the fault and open end of segment C. A fault recorder at the open end of segment C could effectively capture the traveling wave that is isolated between the fault and open end of segment C, and use the traveling wave to estimate the fault location. Similarly, a fault recorder at the end of segment B could effectively capture a simple traveling wave generated from a fault on segment B, and use the traveling wave to estimate the fault location. A complexity occurs when the fault is in section A.
A fault in section A would cause the fault recorders at the open ends of section B and C, to record complex waveforms resulting from multiple reflections. Since a branched circuit could take any form, multiple solutions for the fault location estimation would result. This limits the usefulness of a fault recorder and estimator for branched circuits.